1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly to a method of fabricating a semiconductor device which uses at least two insulating films.
2. Description of the Prior Art
In methods of fabricating a semiconductor device which includes within an impurity diffusion region of a predetermined conductivity type, an impurity diffusion region of the opposite conductivity type, one has heretofore used the graft base transistor process in order, for example, to narrowly form the width of the emitter region of a bipolar transistor. As a concrete example of this process, U.S. Patent Application Ser. No. 517,648 describes a method of fabricating a semiconductor device, which method comprises the steps of forming an oxide film on a silicon substrate, forming a frame-shaped opening mask in the oxide film, performing the first impurity diffusion of high-concentration P-type impurities into the substrate through this mask, forming an oxide film to cover the opening portion, forming the second opening mask in the central mask portion of the frame-shaped opening mask, performing the second impurity diffusion of P-type impurities through the second opening mask, and performing the third impurity diffusion of high-concentration N-type impurities using the second opening mask "as is," so as to form the emitter of the semiconductor device.
With the above-mentioned method, however, it is necessary for the second impurity diffusion or the third impurity diffusion for the emitter region to process the oxide film to make the second opening mask, and a mask mis-register may occur in case of the mask positioning between the second mask and a base region with high concentration as has already been formed by the first impurity diffusion, thus making it impossible to form the emitter width narrowly and precisely.